That's the symbol for a variable resistor, in this case, with a value of 1K Ohm.
Terminal 2 is connected to the "wiper" which sweeps across the resistive material to provide a changing amount of resistance as the wiper is turned (or slid). If B_OUT were connected to terminal 3, it would see the maximum amount of resistance of 1K, equivalent to turning the wiper to the upper extreme.